As shown in FIG. 1, one type of prior art mechanical coupling 10 for joining pipe elements 12 and 14 end to end relies on arcuate projections, known as keys 16 that mechanically engage circumferential grooves 18 in the pipe elements. While these couplings have proved to be very effective and efficient, the prior art configuration is subject to certain limitations. For example, when such a joint is subjected to loads, especially loads arising from internal pressure induced end loads, axial tensile forces and bending, the joint may not be able to withstand such loads up to the full tensile strength of certain types of pipe. To realize a greater percentage of the potential strength of the pipe element and thereby increase the pressure capacity of a joint, external rings containing grooves may be welded to pipe elements to provide for mechanical engagement with the coupling's keys in a configuration that does not alter the pipe element's sidewall, either by removing material (machined grooves) or by deforming the sidewall (rolled grooves).
While welded external rings may permit a larger percentage of the full pipe strength to be realized at a joint, the disadvantage of this solution is the need to weld rings onto the pipe elements. This procedure adds cost, time and requires skilled welders, complicating fabrication. There is clearly a need for a pipe design that improves the realization of pipe element strength and thereby increases the internal pressure performance and axial tensile loading limits achievable using mechanical couplings without the need for external welded rings.